Our data showed that methadone administration during pregnancy in rats decreased male fetal plasma testosterone (T) and delta 4-androstendione (delta 4) levels. Female fetal plasma T and delta 4 levels were unaffected. This study is designed to investigate whether methadone directly or indirectly affects the fetal testes' capacity to synthesize T. Pregnant Sprague-Dawley rats will be treated with various dose levels of methadone from day 15 through 20 of pregnancy, and the fetal testicular tissue will be used for incubation with radiolabeled substrate to determine the capacity of the fetal testes to synthesize T. The labeled T will be obtained by decapitation in the methadone-treated animals, and the blood serum will be used for measuring luteinizing hormone (LH) by the radioimmunoassay technique. The data will be analyzed to determine whether methadone directly affects the steroid-producing enzymes in the fetal testes or whether LH secretion is affected by methadone treatment.